CS52843

http://onsemi.com

6

Figure 5. Startup Voltage for the CS52843

ON/OFF Command to reset of IC

< 15 mA

< 500 μA

Undervoltage Lockout

During Undervoltage Lockout (Figure 5), the output

driver is biased to sink minor amounts of current. The output

should be shunted to ground with a resistor to prevent

activating the power switch with extraneous leakage

currents.

PWM Waveform

To generate the PWM waveform, the control voltage from

the error amplifier is compared to a current sense signal

which represents the peak output inductor current (Figure

increase, thus reducing the duty cycle. This is an inherent

feed−forward characteristic of current mode control, since

the control voltage does not have to change during changes

of input supply voltage.

When the power supply sees a sudden large output current

increase, the control voltage will increase allowing the duty

cycle to momentarily increase. Since the duty cycle tends to

exceed the maximum allowed to prevent transformer

saturation in some power supplies, the internal oscillator

waveform provides the maximum duty cycle clamp as

programmed by the selection of oscillator timing

components.

Figure 6. Timing Diagram

OSC

RESET

Toggle

F/F Output

EA Output

Switch

Current

Setting the Oscillator

tc + RTCT ln

VREF * VLOWER

VREF * VUPPER

td + RTCT ln

VREF * IdRTVLOWER

VREF * IdRT * VUPPER

Substituting in typical values for the parameters in the

above formulas:

VREF + 5.0 V, VUPPER + 2.7 V,

VLOWER + 1.0 V, Id + 8.3 mA

then

tc [ 0.5534RTCT

td + RTCT ln

2.3 * 0.0083RT

4.0 * 0.0083RT

Grounding

High peak currents associated with capacitive loads

necessitate careful grounding techniques. Timing and

bypass capacitors should be connected close to GND in a

single point ground.

The transistor and 5.0 kΩ potentiometer are used to

sample the oscillator waveform and apply an adjustable

ramp to Sense.